The following text should not be construed as an admission of knowledge in the prior art. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference forms a part of the common knowledge in the art.
Proton conducting fuel cells (PCFCs), and other intermediate temperature protonic ceramic electrochemical devices (IT-PCECDs) exhibit several advantages over traditional solid oxide fuel cells (SOFCs) in terms of lower operation temperature (300° C.-600° C.) and higher efficiency. Among proton conducting ceramics, the recently reported proton conductor BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) has shown particularly promising performance in single-cell fuel cell demonstrations at test temperatures around 750° C. However, the maximum power density of the PCFCs achieved (˜1100 mW/cm2 and at 750° C.) was much lower than that of SOFCs.
Very few promising performances for PCFCs at temperatures lower than 600° C. have been reported. One of the challenges for the development of high performance, intermediate temperature PCFCs lies in the discovery of appropriate cathode materials. The poor performance of most PCFCs is attributed, in part, to the use of cathodes that were developed for SOFCs operating at much higher temperatures of between about 700° C. to 1000° C. where as the target PCFC operation temperatures are near 500° C. One reason for this is that the application of conventional SOFC cathodes, which are based on either electron-conducting oxides or mixed oxygen ion and electron-conducting oxides, to electrolytes developed for PCFCs restricts the cathode reaction only to points where the electrolyte phases meet.
Although mixed oxygen ion and electron conducting oxides and proton conducting oxides have been researched extensively, none have been promising. For example, although yttrium-doped barium zirconates (BZY) are excellent proton conductors and also exhibit some oxygen-ion conductivity in dry reducing atmospheres, its electronic conductivity is extremely small. However, it is unquestionable that a prerequisite for a promising intermediate temperature PCFC is high electronic conductivity. Similarly, while BaCo0.4Fe0.4Zr0.2O3-δ (BCFZ) provides a strong electrochemical performance and has stability, making it compatible with BCZYYb electrolytes, the oxygen ion transport in the cathode limits the cells performance.
Accordingly, there exists a need in the art for an intermediate temperature PCFC having a high electronic conductivity, high oxygen ion transport, and compatibility with PCFC electrolytes.